Hydrogen gas (H2) is widely used in industrial and laboratory settings. Because of its high-flamability in air, the need to detect hydrogen gas at levels below its lower explosive limit (LEL is 4% at 25° C.) is of considerable importance. The use of palladium (Pd) as a hydrogen sensor is well-established (F. A. Lewis, “The Palladium Hydrogen System,” Academic Press, New York, 1967.) and based on the increased resistance realized when hydrogen dissolves into the metal creating a palladium hydride which has a lower degree of conductivity than pure palladium.
Recently, a palladium nanowire (also known as a “mesowire,” where mesoscopic structures are characterized by a length scale ranging from the tens of nanometers to micometers) sensor has been described which operates with an inverse response, i.e., it realizes a decreased resistance when exposed to hydrogen (F. Favier, E. C. Walter, M. P. Zach, T. Benter, R. M. Penner “Hydrogen Sensors and Switches from Electrodeposited Palladium Mesowire Arrays,” Science, 293, p. 2227-2231, 2001). Such nanowires are electrodeposited, from solutions of palladium chloride (PdCl2) and perchloric acid (HClO4), onto an electrically-biased graphite step ledge (presumably, these terraced step ledges produce an enhanced field leading to selective deposition). Once formed, these nanowires are transferred to an insulating glass substrate using a cyanoacrylate film. The diameters of these wires are reportedly as small as 55 nanometers (nm) and they possess gaps or break-junctions which impart them with high resistance. When hydrogen is introduced, a palladium-hydride (PdHx) forms. At room temperature (25° C.), there is a crystalline phase change from α to β when the concentration of hydrogen in air reaches 2% (15.2 Torr). Associated with this phase change is a corresponding 3-5% increase in the lattice parameter of the metal which leads to a “swelling” of the nanowire, thus bridging the nanogap breakjunctions (nanobreakjunctions) and leading to an overall decrease in the resistance along the length of the nanowire. The resistance change that occurs is between 6 and 8 orders of magnitude (typical devices see 1×10−11 amps in the “off” state, and 1×10−4 amps in the “on” state). This behavior is unique to nanowires possessing such nanogap breakjunctions. Fortunately, for sensor applications, these gaps re-open when the nanowires are removed from the hydrogen-containing environment, and the swollen nanowires revert back to their pre-swollen state.
The above-mentioned nanowire sensors have three primary deficiencies which can be improved upon. The first deficiency is the reliance on terraced graphite step ledges to form the nanowires. This limits the ability to pattern the nanowires into an arrangement of one's own choosing, i.e., it limits the length and orientation of the nanowires. The second deficiency lies with the need to transfer the nanowires from the conducting graphite surface to an insulating glass substrate using a cyanoacrylate “glue.” Such transfer steps could damage the nanowires. Lastly, there are hydrogen concentration and temperature constraints which present, perhaps, the greatest deficiency in the current state of the art. At 25° C., for example, there is no H2 concentration range over which this sensor can detect-merely a 2% threshold. By 50° C., this threshold moves up to 4-5% H2 in air, which is above the lower explosive limit. Consequently, such nanowire sensors essentially provide only a hydrogen detection capability within a very narrow temperature range.
As a result of the foregoing, there is a need for a method that permits the ordered patterning of nanowires on a surface in a predefined way, and for a method that eliminates the need for the nanowires, once formed, to be transferred to another substrate. There is also a need for a method that overcomes the temperature/threshold concentration limitations of current hydrogen sensors and allows for a range of H2 concentrations to be determined at any given temperature, and which allows for a wider range of operating temperatures such that the sensor is capable of detecting H2 below its lower explosive limit. Such a device could serve as a variable or dynamic-range hydrogen sensor.